This invention relates to electric switches and more particularly to switches which are used to switch very large direct currents such as the currents required for electromagnetic projectile launching.
One type of well known electromagnetic projectile launcher includes a pair of parallel conductive rails, a sliding armature for conducting current between the rails and propelling a projectile along the rails, a source of direct current, and a switching system for directing current from the current source to the projectile launching rails. Current sources which employ an inductive energy storage element and a homopolar generator have demonstrated the capability of providing sufficient current to achieve an acceptable projectile acceleration. However, such inductively driven launcher systems require the service of an opening switch to accomplish the required power compression. The functions performed by the opening switch include: providing, in the closed position, a low resistance path for current flow during the charging of the inductive energy storage device; and commutating, within a short time interval of typically less than one millisecond, the current flow into the conductive rail load. In repetitive firing launcher systems, these functions must be performed in rapid succession.
Inductively driven electromagnetic launchers typically require peak current magnitudes on the order of several hundred thousand to several million amperes. To achieve these current magnitudes, the inductive energy storage device may have to be charged for a time on the order of several tens to several hundreds of milliseconds. This results in an accumulated I.sup.2 t through the switching system during charging of about 10.sup.11 A.sup.2 -sec. per shot. To reduce resistive losses during inductor charging, the switch must be designed with minimum resistance. This generally requires sliding switch contacts with multiple contact points and massive current carrying conductors. The requirement of a massive moving contact presents difficulty in the construction of a switch to perform the current commutation function in which a fast rise of switch voltage is required. Such a fast rise in switch voltage generally can be obtained by parting the switch contacts at high speed, which for massive moving contacts demands large and powerful switch actuating mechanisms.
For a single shot launcher application, a linear parallel conductor switch mechanism such as the switch disclosed in U.S. Pat. No. 4,369,692, issued Jan. 25, 1983 to Kemeny, can successfully provide the switching function. However, such a switch cannot provide rapid repetitive operation. U.S. Pat. No. 4,426,562, issued Jan. 17, 1984 to Kemeny discloses a rotary switch design using a rotor as a bridging contact to make and break with stationary switch terminals. The stationary switch terminals are electrically insulated from each other and constitute a part of the switch housing. With proper timing of the rotor rotation, the switch can provide a low resistance current path for inductor charging and can subsequently break contact to generate two arcs in series to initiate current commutation. To develop the required contact opening speed, rapid acceleration of the switch rotor is required.
The extremely high value of accumulated I.sup.2 t duty placed upon the switch during inductor charging requires massive moving contacts to minimize the resistive loss. A massive moving contact, however, presents difficulties in achieving acceptable contact opening speed needed to perform the fast current commutation into the load. Although this problem may be overcome by accelerating the moving contact well ahead of the initiation of current commutation, the required contact engagement length and/or the kinetic energy consumed in accelerating the moving contact can be unacceptably large. Keeping the massive moving contact at constant linear or rotating speed is not an acceptable solution since this would require a contact engagement length on the order of a few meters to provide a current conducting path during the inductor charging.